Digital Communication 4

DIgItal CommunICatIon
ECE 422l
IV. Bandwidth Utilization

2013

Bandwidth utilization
•Bandwidth is the precious commodity in
communication

•Bandwidth utilization is the wise use of
available bandwidth to achieve
specific goals.

2

Bandwidth utilization

Efficiency can be achieved by
multiplexing;
privacy and anti-jamming can be
achieved by spreading.
3

MULTIPLEXING
• Whenever the bandwidth of a medium linking
two devices is greater than the bandwidth needs
of the devices, the link can be shared.
• Is the set of techniques that allows the
simultaneous transmission of multiple signals
across a single data link.
• As data and telecommunications use increases, so
does traffic.

4

Dividing a link into channels

5

Frequency-division multiplexing

7


FDM is an analog multiplexing technique
that combines narrow band analog
signals.

8

FDM demultiplexing example

10


Applications:
• Broadcasting
• Telephone and data communication
system
• Cable television
• Data distribution networks

11

Example 1

Assume that a voice channel occupies a bandwidth of 4
kHz. We need to combine three voice channels into a link
with a bandwidth of 12 kHz, from 20 to 32 kHz. Show the
configuration, using the frequency domain. Assume there
are no guard bands.

12

Example 1
Solution
We shift (modulate) each of the three voice channels to a
different bandwidth, as shown in next slide. We use the
20- to 24-kHz bandwidth for the first channel, the 24- to
28-kHz bandwidth for the second channel, and the 28- to
32-kHz bandwidth for the third one. Then we combine
them

13

Example 2

Five channels, each with a 100-kHz bandwidth, are to be
multiplexed together. What is the minimum bandwidth of
the link if there is a need for a guard band of 10 kHz
between the channels to prevent interference?

15

Example 2
Solution
For five channels, we need at least four guard bands.
This means that the required bandwidth is at least
5 × 100 + 4 × 10 = 540 kHz,
as shown in next slide

16

Example 3

Four data channels (digital), each transmitting at 1
Mbps, use a satellite channel of 1 MHz. Design an
appropriate configuration, using FDM.

18

Example 3
Solution
The satellite channel limited to 1MHz. We divide
this among the four data channel that is each
channel is allocated with 250 kHz bandwidth.
Since the system is using FDM (analog) and the
input signal is in digital, we are going to used
digital to analog modulation. One solution is 16QAM modulation.
19

Analog hierarchy (AT&T)

Message Channel

21

Example 4
The Advanced Mobile Phone System (AMPS)
uses two bands. The first band of 824 to 849
MHz is used for sending, and 869 to 894 MHz is
used for receiving. Each user has a bandwidth of
30 kHz in each direction. How many people can
use their cellular phones simultaneously?

22

Example 4

Solution
Each band is 25 MHz. If we divide 25 MHz by 30
kHz, we get 833.33. In reality, the band is divided
into 832 channels. Of these, 42 channels are used
for control, which means only 790 channels are
available for cellular phone users.
23

Wavelength Division Multiplexing
• The first commercial use of WDM was to
use different frequencies of light for
transmitters on each end of one fiber.
• The velocity of propagation is equal to the
product of the wavelength and the
frequency
– vp = λ * f
– In a vacuum, the speed of light, C, is 3x108 m/s
24

• Also called Dense wavelength division
multiplexing
• The development of the erbium-doped fiber
amplifier (EDFA) made DWDM possible.
• Easy to do with fiber optics and optical sources
• Give each message a different wavelength
(frequency)
25

• multiplexes multiple data streams onto a single fiber
optic line.(4, 8, 16, 32, and 180 lasers in the
transmitter)
• C band EDFAs operate from 1530 nm to 1560 nm.
• The bandwidth of a C Band system is 4 trillion Hz.
• Frequencies used chosen from the ITU Grid.

26

• Each signal carried on the fiber can be transmitted
at a different rate from the other signals.
• Different wavelength in a light pulse travels
through an optical fiber at different speeds
– Blue light is slower than red light

• Each wavelength takes a different transmission
path

28


29

29


30

Merits

Enhanced capacity - >100 gbps
Full dup;ex transmission
Easier to configure
reliable
Demerits
close wavelength may cause interference(0.8nm)
physical limitation of the fiber
limited to two point circuit or a combination of many
two point circuit
requires mod/demoulator as many as the propagating
wavelenght

31

Time Division Multiplexing

is a digital multiplexing technique for
combining several low-rate
channels into one high-rate one.

32


33

Sharing of the signal is accomplished by dividing available
transmission time on a medium among users.
Digital signaling is used exclusively.
Time division multiplexing comes in two basic forms:
1. Synchronous time division multiplexing, and
2. Statistical, or asynchronous time division multiplexing.

34

Synchronous TDM
The original time division multiplexing.
The multiplexor accepts input from attached devices in a
round-robin fashion and transmit the data in a never ending
pattern.
T-1 and ISDN telephone lines are common examples of
synchronous time division multiplexing.

35

Synchronous TDM

In synchronous TDM, the data rate
of the link is n times faster, and the unit
duration is n times shorter.

37

Synchronous TDM
• If one device generates data at a faster rate than other
devices, then the multiplexor must either sample the
incoming data stream from that device more often than it
samples the other devices, or buffer the faster incoming
stream.
• If a device has nothing to transmit, the multiplexor must
still insert a piece of data from that device into the
multiplexed stream.

38

Synchronous TDM
So that the receiver may stay synchronized with the incoming
data stream, the transmitting multiplexor can insert
alternating 1s and 0s into the data stream.

41

Synchronous TDM
• One eight-bit PCM code from each channel
is called a TDM frame and the time it takes
to transmit one TDM frame is called frame
time and it is equal to reciprocal of sample
rate.

42

Synchronous TDM
Three types popular today:
•T-1 multiplexing (the classic)
•ISDN multiplexing
•SONET (Synchronous Optical NETwork)

43

T1 Digital Carrier System
• North American Telephone standards
• Recognized by ITU-T - Recommendation G.733
• The T1 (1.54 Mbps) multiplexor stream is a continuous
series of frames of both digitized data and voice channels.
• Each channel contains an 8 bit PCM code and is sampled
at 8000 times per second

44


A digital carrier system is a communications system
that uses digital pulse rather than analog signals to
encode
information.
45

T-1 line for multiplexing telephone lines

47

• The multiplexer has 24 independent inputs and one
time-division multiplexed output. The 24 PCM output
signals are sequentially selected and connected
through the multiplexer to the transmission line.
• A transmitting portion of a Channel Bank digitally
encodes the 24 analog channels, adds signaling
information into each channel, and multiplexes the
digital stream onto the transmission medium.
49

The
line
calculated as:

speed

is

Each of the 24 channels contains an eight-bit PCM code and is sampled
8000 times a second. Each channel is sampled at the same rate, but may
not be at the same time.

50

•

Later, an additional bit called the framing bit is added to each frame. The
framing bit occurs once per frame and is recovered at the receiver and its
main purpose is to maintain frame and sample synchronization between
TDM transmitter and receiver.

51

•

As a result of this extra bit, each frame now contains 193 bits and the line
speed for a T1 digital carrier system is 1.544 Mbps. { 193 bits × 8000
frames = 1.544 Mbps} .

52

Superframe TDM Format
• The 8-kbps signaling rate used with the
early digital channel banks was excessive
for signaling on standard telephone voice
circuits.
• Therefore, with modern channel banks, a
signaling bit is substituted only into the
least-significant bit (LSB) of every sixth
frame.
53

• Within each superframe are 12 consecutively numbered
frames (1 to12). The signaling bits are substituted in frames 6
and 12, the MSB into frame 6, and the LSB into frame 12.
Frames 1 to 6 are called the A highway, with frame 6
designated the A channel signaling frame. Frames 7 to 12 are
called the B high way, with frame 12 designated the B
channel signaling frame.

55

• To identify frames 6 and 12, a different framing bit sequence
is used for the odd- and even-numbered frames. The odd
frames (frames 1, 3, 5, 7, 9, and 11) have an alternating 1/0
pattern, and the even frames (frames 2, 4, 6, 8, 10, and 12)
have a 00 1110repetitive pattern.
• As a result, the combined framing bit pattern is
1000 11011100

56

• D4 channel banks time-division multiplex 48 voice-band
telephone channels and operate at a transmission rate of 3.152
Mbps, which is slightly more than twice the line speed for 24channel D1, D2, or D3 channel banks because with D4
channel banks, rather than transmitting a single framing bit
with each frame, a 10-bit frame synchronization pattern is
used.

57

• Line speed is calculated as: total no of bits is 8 bits/channel ×
48 channels = 384 bits/frame An additional 10 bits are added
for frame: so 394 bits/frame. Therefore, line speed of DS-1C
system is 394×8000 = 3.152 Mbps.

58

Fractional T Carrier Service

61

Fractional T Carrier Service
• Fractional T1 systems distribute the channels (i.e., bits) in a
standard T1 system among more than one user, allowing
several subscribers to share one T1line.
• The above figure shows four subscribers combining their
transmissions in a special unit called a data service
unit/channel service unit (DSU/CSU). A DSU/CSU is a digital
interface that provides the physical connection to a digital
carrier network. User 1 is allocated 128 kbps, user 2 - 256
kbps, user 3 - 384 kbps, and user 4 - 768 kbps for a total of
1.536 kbps (8 kbps is reserved for the framing bit).

62

North American Digital
Multiplexing Hierarchy

63

North American DS System
• The basic building block for digital
transmission standards begins with the DS0
signal level.
– One voice equivalent
– 8 bits/sample x 8,000 samples/second

• The DS1 signal has 24 voice equivalents
– 193 bits per frame
• 24 x 8 bits per channel
• 1 framing bit

64

D-Type Channel Banks
• D type Channel Bank refers to the terms used in T1
technology.
• Channel Bank defines the type of formatting that is required
for transmission on T1 trunk.
• The purpose of a Channel Bank in the telephone company is to
form the foundation of multiplexing and de multiplexing the
24 voice channels (DS0).
• D type Channel Bank is one of the types of Channel Bank
which is used for digital signals.
• There are five kinds of Channel Banks that are used in the
System: D1, D2, D3, D4, and DCT (Digital Carrier Trunk).

65

Multiplexing hierarchy

66

• A special device called muldem (multiplexers/
demultiplexer) is used to upgrade from one level in
the hierarchy to the next-higher level.
• They handle bit-rate conversions in both directions
and are designated as M12, M23 etc. which identifies
the respective input and output digital signals.
• As shown, an M12 muldem interfaces DS-1 and DS-2
digital signals.

67

• As shown, an M12 muldem interfaces DS-1 and DS-2
digital signals.
• Digital signals are routed at central locations called
digital cross-connects (DSX), which are convenient
for making patchable interconnections and routine
maintenance and troubleshooting. Each digital signal
(i.e. DS-1, DS-2, etc) has its own digital switch
(DSX-1, DSX-2/).
68

T1 Carrier System
• T1 carrier systems were designed to combine
PCM and TDM techniques for the transmission of
24 64-kbps channels with each channel capable of
carrying digitally encoded voice band telephone
signals or data. The transmission bit rate (line
speed) for a T1 carrier is 1.544 Mbps.
• All 24 DS-0 channels combined has a data rate of
1.544 Mbps; this digital signal level is called DS1. Therefore T1 lines are sometimes referred to as
DS-1 lines.
71

T2 Carrier System
• T2 carriers time-division multiplex 96 64-kbps
voice or data channels into a single 6.312 Mbps
data signal for transmission over twisted-pair
copper wire upto 500 miles over a special
LOCAP (low capacitance) metallic cable.
• Higher
transmission
rates
make
clock
synchronization even more critical.

72

T3 Carrier System
• T3 carriers time-division multiplex 672 64kbps voice or data channels for transmission
over a single 3A-RDS coaxial cable. The
transmission bit rate is 44.736 Mbps and
coding technique used with T3 carriers is
binary three zero substitution (B3ZS).

73

T4M Carrier System
• T4M carriers time division multiplex 4032
64-kbps voice or data channels for
transmitting
over a single T4M coaxial cable upto 500
miles. The transmission rate is very high
(274.16kbps) making substituting patterns
impractical. They transmit scrambled
unipolar NRZ digital signals.
74

T5 Carrier System
• T5 carriers time-division multiplex 8064
64-kbps voice or data channels and
transmits them at 560.16 Mbps over a single
coaxial cable.

75

European TDM 30 + 2 System
• In Europe, a different version of T carrier lines is used called E
lines.
• With the basic E1 system, a 125µs frame is divided into 32
equal time slots.

76

• The European TDM system multiplexes 32
DS0 channels together.
– Channel 0 is used for synchronizing (framing) and
signaling.
– Channels 1-15 and 17-31 are used for voice.
– Channel 16 is reserved for future use as a signaling
channel.
– Time slot 17 is used for a common signaling channel
(CSC).

77

• The total signal rate is 2.048 Mbps (64 kbps * 32 channels)
• The signalling for all 32 voice-band channels is accomplished
on the common signalling channel. Consequently, 32 voiceband channels are time-division multiplexed into each E1
frame.

And the line speed can be given as 256 bits/frame × 8000
frames/second = 2.408 Mbps

78


79

Integrated Service Digital Network
•Is a telephone system which provide digital telephone and
data services.
•designed to provide access to voice and data services
simultaneously
•there is no digital to analog conversion.
•Developed by CCITT (Comate Consultative International
Telephonique Telegraphs) to limitation of POTS (Plane old
Telephone system).

80

ISDN frame
The ISDN multiplexor stream is also a continuous stream of
frames. Each frame contains various control and sync info.

81

Channels of ISDN

1. B Channel
• Carries voice, data, video etc.
• functions at a constant 64
kbps.
• can be used for packet and
circuit switching applications.
82

Channels of ISDN

2. D Channel (Denial)
•is used to convey user signaling
massages.
•used out of band signaling .
This means that network related signals are
carried on a separate channel than used data.

83

Channels of ISDN

3. H Channel
•Have a considerably higher
transfer rate than B channels.
•sustains rates of approximately
1920 mbps.
effectively meet the needs of real time video conferencing, digital quality
audio and other services requiring a much higher bandwidth.

84

Use of ISDN

•
•
•
•
•
•

Electronic library Inter connection.
Electronic resources accessing.
Images, sound and video retrieval.
Video conferencing.
Call center.
Internet Access.
85

Synchronous Optical Network

SONET was developed by ANSI;
SDH was developed by ITU-T.

86

SONET Standards
• Fiber optics use synchronous optical
network (SONET) standards.
• The initial SONET standard is OC-1, this
level is known as synchronous transport
level 1 (STS-1).
– It has a synchronous frame structure at a speed
of 51.840 Mbps.
– OC-1 is an envelope containing a DS3 signal
(28 DS1 signals or 672 channels).
87

SONET
• Used in massive data rates

88

SONET

SONET defines four layers:
path, line, section, and photonic.

90

SONET FRAMES

Each synchronous transfer signal STS-n is composed of
8000 frames. Each frame is a two-dimensional matrix of
bytes with 9 rows by 90 × n columns.

93

An STS-1 and an STS-n frame

94

SONET
A SONET STS-n
signal is transmitted at
8000 frames per second.

Each byte in a SONET frame can carry a digitized voice channel.

95

Statistical TDM
• improves the efficiency of a TDM system.
– Channel units do not have reserved time slots.
– Time slots are dynamically assigned.

• Also called stat muxs, intelligent
multiplexers, and asynchronous
multiplexers.

97

Statistical TDM
• A statistical multiplexor transmits only the data from
active workstations (or why work when you don’t have to).
• If a workstation is not active, no space is wasted on the
multiplexed stream.
• A statistical multiplexor accepts the incoming data streams
and creates a frame containing only the data to be
transmitted.

98

Statistical TDM
To identify each piece of data, an address is included.

100

Statistical TDM
If the data is of variable size, a length is also included.

101

Statistical TDM
More precisely, the transmitted frame contains a collection
of data groups.

102

Statistical TDM
A statistical multiplexor does not require a line over as high a
speed line as synchronous time division multiplexing since
STDM does not assume all sources will transmit all of the
time!
Good for low bandwidth lines (used for LANs)
Much more efficient use of bandwidth!

103

Example 5

• The data rate for each one of the 3 input
connection is 1 kbps. If 1 bit at a time is
multiplexed (a unit is 1 bit),
What is the duration of:
(a) each input slot,
(b) each output slot,
(c) each frame?
105

Example 5
Solution
a. The data rate of each input connection is 1

kbps. This means that the bit duration is 1/1000
s or 1 ms. The duration of the input time slot is
1se
1 ms (same as bit duration).
c/
1
000

(3signals * 1 kbps)/ 3 channels = 1 kbps

107

bits

Example 5 (continued)
b. The duration of each output time slot is onethird of the input time slot. This means that the
duration of the output time slot is 1/3 ms.
1/3 ms per channel

1 ms

108

c. Each frame carries three output time slots. So

the duration of a frame is 3 × 1/3 ms, or 1 ms.
“The duration of a frame is the same as the
duration of an input unit.“

1 ms

109

Example 6

Consider a synchronous TDM with a data
stream for each input and one data stream
for the output. The unit of data is 1 bit.
Find :
(a) the input bit duration,
(b) the output bit duration,
(c) the output bit rate,
(d) the output frame rate.
110

Example 6
Solution
a. The input bit duration is the inverse of the bit rate: 1/1
Mbps = 1 μs.
b. The output bit duration ( is one-fourth of the input bit
duration, or ¼ μs.

112

c. The output bit rate is the inverse of the output bit
duration or 1/(4μs) or 4 Mbps.
This can also be deduced from the fact that the output
rate is 4 times as fast as any input rate;
so the output rate = 4 × 1 Mbps = 4 Mbps.
d. The frame rate is always the same as any input rate.
So the frame rate is 1,000,000 frames per second.
113

Example 7
Four 1-kbps connections are multiplexed together.
A unit is 1 bit.
Find:
(a) the duration of 1 bit before multiplexing,
(b) the transmission rate of the link,
(c) the duration of a time slot
(d) the duration of a frame.
114

Example 7
Solution
We can answer the questions as follows:
a. The duration of 1 bit before multiplexing is:
1 / 1 kbps, or 0.001 s (1 ms).
b. The rate of the link is 4 times the rate of a
connection, or 4 kbps.
115

c. The duration of each time slot is one-fourth of

the duration of each bit before multiplexing, or
1/4 ms or 250 μs.
Note that we can also calculate this from the data
rate of the link, 4 kbps. The bit duration is the
inverse of the data rate, or 1/4 kbps or 250 μs.
116


d. The duration of a frame is always the same as
the duration of a unit before multiplexing, or 1
ms.
another way:
Each frame in this case has four time slots. So the
duration of a frame is 4 times 250 μs, or 1 ms.
117

Example 8
Four channels are multiplexed using TDM. If each
channel sends 100 bytes /s and we multiplex 1 byte per
channel,
•show the frame traveling on the link,
•the size of the frame,
•the duration of a frame,
•the frame rate,
•the bit rate for the link.
119

Example 8

Solution
Each frame carries 1 byte from each channel;
•the size of each frame = 4 bytes, or 32 bits.
•each channel = 100 bytes/sec and a frame = 1 byte
•frame rate must be 100 frames / sec.
•The bit rate is 100 × 32, or 3200 bps.

120

Example 9
A multiplexer combines four 100-kbps channels using a
time slot of 2 bits.
•What is the frame rate?
•What is the frame duration?
•What is the bit rate?
•What is the bit duration?

122

Example 9
Solution
•The frame rate is :
(4 channels * 100 kbps)/ (2 bits/channel* 4 channel)
= 50,000 frames per second.
•The frame duration is: 1sec/50,000 frames = 20 μs.
• the bit rate is: 50,000 frames/s × 8bit/frame
= 400,000 bits/s.
•The bit duration is: 1sec /400,000 bits = 2.5 μs.

123

Multiple-slot multiplexing

127

Synchronization
• To ensure that the receiver correctly reads
the incoming bits, i.e., knows the incoming
bit boundaries to interpret a “1” and a “0”, a
known bit pattern is used between the
frames.
• The receiver looks for the anticipated bit
and starts counting bits till the end of the
frame.
129

Synchronization
• Then it starts over again with the reception
of another known bit.
• These bits (or bit patterns) are called
synchronization bit(s).
• They are part of the overhead of
transmission.

130

Example 10
We have four sources, each creating 250 8-bit characters
per second. If the interleaved unit is a character and 1
synchronizing bit is added to each frame, find (a) the data
rate of each source, (b) the duration of each character in
each source, (c) the frame rate, (d) the duration of each
frame, (e) the number of bits in each frame, and (f) the
data rate of the link.

132

Example 10
Solution
a. The data rate of each source is 250 × 8 = 2000
bps = 2 kbps.
b. Each source sends 250 characters per second;
therefore, the duration of a character is 1/250 s,
or 4 ms.
133

c. Each frame has one character from each source,
which means the link needs to send 250 frames per
second to keep the transmission rate of each source.
d. The duration of each frame is 1/250 s, or 4 ms. Note
that the duration of each frame is the same as the
duration of each character coming from each source.
e. Each frame carries 4 characters and 1 extra
synchronizing bit. This means that each frame is
4 × 8 + 1 = 33 bits.
134

Example 11

Two channels, one with a bit rate of 100 kbps and
another with a bit rate of 200 kbps, are to be
multiplexed. How this can be achieved? What is
the frame rate? What is the frame duration?
What is the bit rate of the link?

135

Example 11
Solution
We can allocate one slot to the first channel and two slots
to the second channel. Each frame carries 3 bits. The
frame rate is 100,000 frames per second because it carries
1 bit from the first channel.
The bit rate is 100,000 frames/s × 3 bits per frame, or 300
kbps.

136

Code Division Multiplexing
• Old but now new method
•

Also known as code division multiple access (CDMA)

•

An advanced technique that allows multiple devices to
transmit on the same frequencies at the same time using
different codes

•

Used for mobile communications

137

• An advanced technique that allows multiple devices to
transmit on the same frequencies at the same time.
• Each mobile device is assigned a unique 64-bit code (chip
spreading code)
• To send a binary 1, mobile device transmits the unique
code
• To send a binary 0, mobile device transmits the inverse of
code

138


• Receiver gets summed signal, multiplies it by receiver
code, adds up the resulting values
• Interprets as a binary 1 if sum is near +64
• Interprets as a binary 0 if sum is near –64

139

Summary
• Multiplexing
• Types of multiplexing
– TDM
• Synchronous TDM (T-1, ISDN, optical fiber)
• Statistical TDM (LANs)

– FDM (cable, cell phones, broadband)
– WDM (optical fiber)
– CDM (cell phones)
141

SPREAD SPECTRUM

In spread spectrum (SS), we combine signals from
different sources to fit into a larger bandwidth, but
our goals are to prevent eavesdropping and
jamming. To achieve these goals, spread spectrum
techniques add redundancy.

142

History of Spread Spectrum
Spread Spectrum was actually
invented by 1940s Hollywood
actress
Hedy Lamarr(19132000).
An Austrian refugee, in 1940 at
the age of 26, she devised
together with music composer
George Antheil a system to stop
enemy detection and jamming of
radio controlled torpedoes by
hopping around a set of
frequencies in a random fashion.
143

History of Spread Spectrum
She was granted a patent in
1942 (US pat. 2292387) but
considered
it
her
contribution to the war
effort and never profited.
Techniques known since
1940s and used in military
communication
systems
since 1950s.
144

Introduction to Spread Spectrum
“Spread” radio signal over a wide frequency range
Several magnitudes higher than minimum requirement
Gained popularity by the needs of military communication
Proved resistant against hostile jammers
Ratio of information bandwidth and spreading bandwidth is
identified as spreading gain or processing gain
Processing gain does not combat white Noise

145

SPREAD SPECTRUM
applications:
• able to deal with multi-path
•

multiple access due to different spreading sequences

•

spreading sequence design is very important for
performance

•

low probability of interception

•

privacy

•

anti-jam capabilities

146

Spread Spectrum Applications
Interference
̶ Prevents interference at specific frequencies
̶ E.g. other radio users, electrical systems
Military
̶ Prevents signal jamming
̶ Scrambling of ‘secret’ messages
̶ gps

147

Spread Spectrum Applications
Wireless LAN security
̶ Prevents ‘eavesdropping’ of wireless links
̶ Prevents ‘hacking’ into wireless LANs
CDMA (Code Division Multiple Access)
̶ Multiple separate channels in same medium using
different spreading codes

148

Spread Spectrum Criteria
A communication system is considered a spread spectrum
system
if it satisfies the following two criteria:

Bandwidth of the spread spectrum signal has to be
greater than the information bandwidth. (This is also
true for frequency and pulse code modulation!)
The spreading sequence has to be independent from the
information. Thus, no possibility to calculate the
information if the sequence is known and vice versa.
149

Spread Spectrum Classification

150

Spread Spectrum Classification

151

Direct Sequence Spread Spectrum

•
•

•

Information signal is directly modulated
(multiplicated) by a spreading sequences
Commonly used with digital modulation schemes
The idea is to modulate the transmitter with a bit
stream consisting of pseudo-random noise (PN) that
has a much higher rate than the actual data to be
communicated
Near/far effect
Require continuous bandwidth
152

Direct Sequence Spread Spectrum
•

The use of the high-speed PN sequence results in an
increase in the bandwidth of the signal, regardless of
what modulation scheme is used to encode the bits into
the signal

153

Frequency Hopping Spread Spectrum

The information signal is transmitted
on different frequencies
Time is divided in slots
Each slot the frequency is changed

155

Frequency Hopping Spread Spectrum

The change of the frequency is referred
to as slow if more than
one bit is
transmitted on one frequency, and as
fast if one bit is transmitted over
multiple frequencies
The frequencies are chosen based on the
spreading sequences
156

Frequency selection in FHSS

158

Time Hopping Spread Spectrum
Time divided into frames; each TF long
Each frame is divided in slots
Each wireless terminal send in exactly one
of these slots per frame regarding the
spreading sequence
No near far effect

161

Time Hopping Spread Spectrum

162

Comparison of different Spread Spectrum
Techniques
SS Technique
Direct Sequence

Frequency
Hopper

advantage
• best behavior in multi
path rejection
• no synchronization
• simple implementation
• difficult to detect
• no need for coherent
•
bandwidth
• less affected by the near
far effect

163

disadvantage

• near far effect
• coherent bandwidth

• complex hardware
• error correction
•
needed

Comparison of different Spread Spectrum
Techniques
SS Technique

Time Hopper

advantage
• high bandwidth
efficiency
• less complex
hardware
• less affected by the
near far effect

164

disadvantage
• error correction
needed

Code Division Multiple Access
• analog communication systems used frequency
division multiplexing
• digital systems have employed time division
multiplexing
• to combine many information signals into a single
transmission channel from different sources, these
two methods become frequency division multiple
access (FDMA) and time-division multiple access
(TDMA), respectively.
165

Code Division Multiple Access
• Spread-spectrum communication allows a
third method for multiplexing signals from
different sources:
code-division multiple access
• allow multiple users to use the same
frequency using separate PN codes and a
spread-spectrum modulation scheme
166

Reference
• Digital Communication
– by Sanjay Sharma

• Advance Electronic Communication
– by Robert Tomasi

• World Wide Web
• Slideshare.net
167

Digital Communication 4

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